A Milford man was sentenced to 18 years in federal prison Friday for his conviction on charges related to the production, distribution, and possession of child pornography. Robert Diduca, 48, was sentenced in US District Court in Worcester in a case that sparked an international investigation into child pornography involving at least 138 children, federal officials said. Photos found in Diduca’s possession led to 33 arrests worldwide, said Bruce Foucart, special agent in charge of homeland security investigations for US Immigration and Custom’s Enforcement. Investigators found 27,000 images of child pornography in Diduca’s collection.